Blow molding machines are used to form hollow, plastic articles such as milk bottles and other liquid containers. The blow molded article is formed in a mold that is constructed from a pair of mold halves which are clamped together around an extruded section of softened thermoplastic resin (referred to as parison). After clamping, a blow pin is inserted into an opening of the mold and air is then forced through the blow pin into the mold to expand the parison so that it conforms to the surface of the mold cavity. Movement of the blow pin into and out of the opening in the mold cavity is typically done automatically using a blow pin assembly that operates under PLC control.
On shuttle blow molding machines, the mold is usually moved, or shuttled, between an extrusion work station and a blow pin workstation. At the extrusion workstation, an extruder head extrudes a tube of parison between the two halves of an open mold. The mold is then clamped shut around the parison and the mold is transported to the blow pin workstation where a blow pin moves downward and into engagement with the opening in the mold cavity, with the blow pin extending slightly into the center of the tube of parison. Air is then blown into the parison to expand it against the sides of the mold cavity. See, for example, U.S. Pat. No. 4,421,472, issued Dec. 20, 1983 to Merritt W. Martin, Jr.
Sometimes, the rim of the tube of parison collapses or gets folded over, thereby preventing the blow pin from entering into the center of the parison. As a result, the parison is not properly expanded in the mold and the resulting article must be rejected. To avoid this problem, the blow pin has sometimes been used to pre-blow the parison; that is, air is forced through the blow pin as it is being brought down into engagement with the mold to help hold the tube of parison open.
On rotary blow molding machines, the molds are located on a carrousel that indexes the molds in a circle through different workstations. The molds are paused at each workstation for a different operation such as, for example, extrusion of the parison into the mold at an extrusion workstation. On these types of blow molding machines, the blow pin assembly may be mounted on the carrousel so that it indexes with its associated mold. Typically, this is done by mounting the blow pin assembly on the mold clamp so that it moves with one of the two halves of the mold clamp. However, since the blow pins are indexed around through the workstations along with their associated molds, the blow pin assembly must not only move the blow pin into and out of the mold opening, but must also retract the blow pin away from above the opening. Otherwise, the blow pin would interfere with the extrusion head when it dispenses parison at the extrusion workstation.
As a result, blow pin assemblies for rotary blow molding machines have been designed to move the blow pin along an arcuate path between a retracted, or home, position and a mold-engaging position. See, for example, U.S. Pat. No. 4,919,607, issued Apr. 24, 1990 to M. Warren Martin et al. However, one problem with these types of blow pin assemblies is that, because of the arcuate path, the blow pin does not enter the opening along a purely linear path (i.e., along the central axis of the mold cavity that extends through the opening). While the Martin et al. patent utilizes a pivotal linkage that minimizes the arcuate travel of the blow pin as it approaches and enters the opening in the mold, the movement of the blow pin nonetheless has a horizontal component to it that can result in the blow pin catching the upper edge of the parison and folding it over the opening in the tube of parison, making proper expansion of the parison impossible. Also, with this type of blow pin assembly it may not be possible to pre-blow the parison because the blow pin is not entering from directly above the tube of parison and doing so can again fold over the upper edge of the parison.
Accordingly, there exists the need for a blow pin assembly for a rotary blow molding machine that reduces the possibility of the blow pin catching and folding over the edge of the parison and that permits the parison to be pre-blown to better improve the yield of the machine.